In the field of wireless communication, transmitted signals and received signals are in general radio-frequency (RF) signals, and processed signals are all low-frequency signals. Thus, in the structure of an RF transceiver, a mixer is needed to convert low-frequency signals to RF signals that are then transmitted through an antenna. Further, a mixer is also needed to convert received RF signals to low-frequency signals, which are then processed by a backend digital circuit of the transceiver. A mixer circuit need a reference frequency in order to determine whether to convert RF signals in a predetermined frequency to low-frequency signals or to convert low-frequency signals in a predetermined frequency to RF signals. This reference frequency is provided by a voltage controlled oscillator (VCO) in the structure of the RF transceiver. Such VCO in the field of RF communication usually selects a form of transistor cross coupling pairs to provide energy needed for oscillations of a circuit.
In a semiconductor manufacturing process, production variations are constantly monitored due to delay effects caused by inductors and capacitors. A common solution is installing an additional ring oscillator in a manufacturing equipment. However, to reduce the cost of ring oscillators, multiple cascaded inverters are usually adopted.
For example, the Taiwan Patent No. 1296877 discloses a conventional oscillator. The oscillator of the above disclosure, operable in a variable driving voltage to generate an oscillation signal having a predetermined period, includes a plurality of logic elements connected to one another in a ring. Each logic elements includes a plurality of inverters. The oscillator further includes a voltage generating device that generates a first driving voltage and a second driving voltage, which are selectively applied to the logic element. Selectively applying the first driving voltage and the second voltage to the logic element causes an effect on the period of a signal that the oscillator generates. In a normal operation, when it is discovered that the period of a signal is equal to a predetermined period through a test, the first driving voltage is applied to the logic element; when it is discovered that the period of the signal of the oscillator differs from the predetermined period, the second driving voltage is applied to the logic element.
In the above prior art, the conventional ring oscillator disfavors the operation of baseband signals in a high frequency band. One reason is that, when the number of cascades in the ring oscillator increases, the phase margin (PM) generated is insufficient for maintaining oscillation conditions in the high frequency band. One solution for overcoming this issue is reducing to fundamental frequency. In other words, as the number of cascades increases, the lower oscillation frequency that the oscillation circuit generates becomes, hence limiting the operating frequency of the conventional ring oscillator.